Demonstrators in the Philippines protest against the IMF-World Bank meetings being held in Singapore. Photograph: Bullit Marquez/AP

More than 80 non-governmental organisations announced they were boycotting the International Monetary Fund and World Bank annual meetings that began today in Singapore, in protest at the host government banning at least 27 accredited activists and many others from entering the country.

The NGOs were to make a formal statement on Friday but organisers told the Guardian this afternoon that 80 groups had already agreed to the boycott and more were expected.

"We estimate this means at least half of the 500 NGO activists due to attend the meetings will not do so," said Donartus Marut, of the International NGO Forum on Indonesian Development.

Oxfam, Greenpeace and the World Development Movement (WDM) are among the more prominent organisations now avoiding the official meetings, although some still intend to go to Singapore in an unofficial capacity or hold events away from the official venue.

"We're joining the boycott to show solidarity with our partners and allies who were denied access to exercise their fundamental rights, through freedom of expression and association, to attend the meeting," said Taylor Thompson of Oxfam.

Among the 27 activists is a Briton, Martin Powell, of WDM. Murray Benham, WDM's head of campaigns, said he could not understand why the organisation was banned.

"We've clearly got a long history of criticising the bank and the fund and recently issued a report calling for the bank and fund's abolition and replacement with other mechanisms," he told the Guardian. "But you wouldn't have thought a fairly academic report would put us beyond the bounds of acceptance."

Singaporean police have said the banned activists pose a threat to security, law and order. They have banned all outdoor demonstrations in connection with the meetings and have designated an area the size of a volleyball court for indoor protests.

In the past week the World Bank and IMF have repeatedly issued strong statements condemning Singapore's decision.

The World Bank's president, Paul Wolfowitz, today told about 50 activists that the ban meant Singapore was reneging on a three-year deal.

"The most unfortunate thing is what appears to be a going-back on an explicit agreement," he said. "So far we've had no satisfactory explanation why."

Mr Wolfowitz is scheduled to meet senior Singaporean ministers later today and is expected to protest against the ban.

Shortly before the meeting the government softened its position slightly, saying it might allow the activists in if they travelled to Singapore.

"We will assess at the point of entry whether they pose a security or safety risk," the Singapore organising committee for the event said in a statement. "If we judge the risk to be acceptable for that particular activist, we are prepared to allow him or her in. However, we cannot guarantee that all 27 activists will be admitted to Singapore."

Singapore has also banned the holding of a civil society forum that traditionally is held in parallel to the official meetings. Some 800 NGO activists are instead gathering on the neighbouring Indonesian island of Batam from tomorrow until Sunday.

Following alleged pressure by the Singaporean authorities the Indonesian police have ordered all outdoor events at the forum to be cancelled.